Numerous industries, such for example as the chemical industry, the paper industry or the food industry, throw away large amounts of liquids charged with insoluble mineral or organic materials. For recycling these liquids or disposing of them without polluting the environment, they are in general subjected to previous clarification, this treatment being generally carried out in decanters or flotation apparatus of different designs which provide on the one hand a clarified liquid and on the other hand a suspension of solid particles in the form of sludges which may still contain up to 98% of liquid.
For recycling the solid particles or else for facilitating the handling and/or the transport of the sludges, it is desirable to reduce very substantially the liquid content of these latter, this result being obtained more especially by using the above defined apparatus.
In known apparatus of this type, the screw, because of its rotational movement, causes the suspension to be transferred from the rear end to the front end of the cylinder, i.e. towards the thickened suspension outlet at the level of which it forms, possibly in cooperation with appropriate means, a plug which prevents the liquid separated during the thickening from being discharged through this outlet and brakes to a greater or a lesser degree the discharge of the thickened suspension which is thus compacted to a greater or lesser degree. During this transfer, the suspension is pushed on the thrust face of the threads of the screw, which results in a separation of the liquid and so progressive thickening of the suspension, the separated liquid being discharged to the outlet provided for this purpose. However, in these apparatus, the pressure exerted by the screw on the suspension remains substantially constant during the whole thickening procedure, i.e. during the whole time when the suspension progresses from the rear end to the front end of the cylindrical enclosure. It will therefore be readily understood that, although concentration of the sludges is relatively easy in the rear part of the cylindrical enclosure where the viscosity of said sludges is low, this concentration is more and more difficult to obtain as the viscosity of the sludges increases, that is to say as these latter progress in the enclosure. The result is necessarily a limitation of the degree of thickening of the sludges leaving the apparatus.